Home pregnancy test

The assay is carried out on a test strip and based on a sandwich format with two antibodies. One antibody, the capture antibody, is immobilised, i.e. covalently attached to the device surface. The second antibody, the tracer antibody, is labelled, usually with a dye. This tracer antibody is impregnated onto the surface of the device, but is not permanently attached. The strip component is composed of an adsorbent material. Once the urine sample is applied, the liquid moves along the strip by capillary action and the assay-reactions are carried out in flow. 

A schematic of a test strip is shown in Fig. 5.12. The adsorbent material is usually enclosed within a plastic casing featuring a sample input window, a test result window and a control window. A drop of urine is applied at the sample input and the liquid first moves over the zone, which contains the labelled tracer antibody (Fig. 5.13). If hCG is present in the sample, it forms a complex with the tracer antibody. This complex continues to move along the adsorbent material and passes over the area with the immobilised capture antibody. A sandwich is formed between the immobilised capture antibody and the tracer antibody with the hCG in between. Thus, the initially mobile antibody with the label becomes immobilised. The amount of sandwich complexes formed is directly proportional to the amount of hCG present in the sample. If the hCG concentration exceeds a minimum concentration, the dye colour becomes visible to the eye. 

In Fig. 5.14, positive and negative test results are depicted for an assay using a red dye as a label. The control window always shows the colour of the label if the 

Other assays commonly used for home-testing include cholesterol, blood-glucose, urine-glucose, blood type and alcohol-tests. 

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Latex agglutination immunoassays are easily formatted into simple kits which can provide yes/no and semiquantitative estimates of antigen (or antibody) in a sample. The first such assay was developed in 1957 for rheumatoid factor (15) and assays are on the market for the deterrnination of many species of bacteria, fungi. Mycoplasma, parasites, ckettsia, and vimses, as well as for the deterrnination of autoimmune disease, hormones (qv), dmgs (see Pharmaceuticals), and blood proteins (16). Latex agglutination is also the basis of many home pregnancy tests. 

Because the sheer magnitude of the tests performed in laboratory diagnosis is so great, e.g., over 3,600 tests in the alphabetical index of the laboratory tests offered by UCLA Medical Center [4], a single example will be illuminated to demonstrate a particular use of biologically derived diagnostics in home pregnancy test. 

LC is a 28-year-old woman who comes to your practice because she thinks that she is pregnant. She reports that she has not had a period in 9 weeks. When she first missed her period just over 1 month ago, she was not concerned because her cycle had become very light. However, when she missed her cycle again, she took a home pregnancy test. That test was positive. In your practice, a repeat urine pregnancy test confirms that the patient is pregnant. 

Dyed particles also are commonly used in diagnostic lateral flow tests (like the common home pregnancy test), as the colors can be seen with the eye without the need for special detectors. In this type of assay, antibodies or antigens are coupled to the dyed particles and a sample solution applied to the test strip carries them along within a membrane. The particles then are captured at points in the membrane that represent either a control or a positive sample result. Large numbers of color particles docking at these points within the membrane create the visual lines associated with these disposable tests. 

Figure 10.1 Home pregnancy testing devices, like the one seen here, utilize biotechnology techniques. This test uses paired monoclonal antibodies and colored beads to indicate increased human gonadotropin in urine. The second round well is a control, indicating the device is working properly by trapping a common urine protein.

Bastian, L.A., K. Nanda, V. Hasselblad, and D. L. Simel. Diagnostic Efficiency of Home Pregnancy Test Kits A Meta-analysis. Archives of Eamily Medicine 7 no. 5 (1998) 465-469. 

Butler SA, Khanliaii SA, Cole LA. Detection of early pregnancy forms of human chorionic gonadotropin by home pregnancy test devices. Clin Chem 2001 47 2131-6. 

Cole LA, Khanlian SA, Sutton JM, Davies S, Rayburn WE. Accuracy of home pregnancy tests at the time of missed menses. Am J Obstet Gynecol 2004 190 100-5. 

Hicks JM, losefsohn M. Reliability of home pregnancy-test kits in the hands of laypersons. N Engl J Med 1989 320 320-1. Erratum in N Engl J Med 1989 321 193. 

Jeng LL, Moore RM Jr, Kaczmarek RG, Placek PJ, Bright RA. How frequently are home pregnancy tests used Results from the 1988 National Maternal and Infant Health Survey Birth 1991 18 11-13. 

Medical devices can be anything from thermometers to artificial hearts to in-home pregnancy test kits. Devices, unlike drugs, are not dependent on a chemical action. Device inventors are more concerned with anatomy—skin, internal organs, tissue—and the compatibility of the device both within and on the surface of the body. 

Class II—Special Control Class-II devices, such as motorized wheelchairs, infusion pumps, surgical drapes, and some home pregnancy test kits, require additional regulation in order to ensure proper design and performance standards. Such devices may require special labeling and postmarket surveillance. Most medical devices fall into this category. 

J. M. Hicks and M. losefsohn. Reliability of Home Pregnancy-Test Kits in the Hands of Laypersons, New England Journal of Medicine, vol. 320, no. 5, pp. 320-321, 1989. 

One of the earliest such POC tests developed was the home pregnancy test. It is based on a qualitative immunological antibody assay. It relies on the determination of human chorionic gonadotrophin (hCG) levels in urine, which gives rise to a coloured band if positive. The device is very small and completely disposable. A quality control check is built in where the rest of the urine sample continues along the dipstick and interacts with a pH-sensitive indicator to show that the test has been performed correctly. 

With the increased availability of a variety of home diagnostic tests, the sale of home pregnancy, ovulation, and fertility tests alone was a 330 million dollar business in 2005. Home pregnancy tests have been utilized since 1976, with approximately 26 devices currently available in the United States. These tests claim that they can be utilized on the first day of a missed menstrual period (or, for oertain devioes, even earlier) with more than 99% diagnostic accuracy for pregnancy. Despite these claims, accurate results are obtained only a fraction of the time (Table 46.1) (1). 

Table 46.1. Clinical Sensitivity Evaluation of Home Pregnancy Tests (1) Name of Device Confidence in Positive Results (%)Clinical Sensitivity (%) ...

Table 46.2. Evaluation of Home Pregnancy Tests with Regular Human Chorionic Gonadotropin (hCG) and Hyperglycosylated hCG (H-hCG) Standards (2) ...

Hulisz DT, Urbanski CM. Home pregnancy testing kits. US Pharmacist 2002 21 22-32. 

Most blood proteins do not show up in the urine, but hCG does. And it is produced very soon after the egg is fertilized, and then in increasing amounts as the pregnancy progresses. Sandwich ELISA (see Figure 4.35 in the text) is the ideal method for complex biological fluids, and it is relatively easy to produce two different monoclonal antibodies to epitopes on opposite sides of the protein. All home pregnancy test kits are based on variations of this method. 

For a better understanding of the use of ELISA in home pregnancy tests, view the Animated Technique Elisa Method for Detecting HCG at www.whfreeman.com/biochem5. 

Encourage the client to confirm with a home pregnancy test. 

The client can perform a home pregnancy test, but it is not the first intervention the nurse should discuss with the client. 

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